Malaria is a mosquito-borne disease caused by a parasite. Plasmodium falciparum and Plasmodium vivax are the two predominant human malaria parasites. Although malaria was eliminated as a major public health problem in the United States in the late 1940's, it remains a major health problem in developing countries. Each year, 350-500 million cases of malaria occur worldwide and over one million people die, most of them young children in sub-Saharan Africa. Malaria also takes a high toll on pregnant women.
In nature, malaria parasites spread by infecting successively two types of hosts: a vertebrate host (humans) and an invertebrate host (female Anopheles mosquitoes). Malarial parasites enter a human when an infected female mosquito feeds. The parasites, which are called sporozoites at this stage, migrate to the liver where they grow and multiply in hepatocytes, and are released as merozoites. Merozoites infect erythrocytes, where they develop and multiply. In the erythrocytic cycle, the parasite progresses through a series of blood stages (ring stage, trophozoite, and schizont). In the schizont stage, the infected erythrocyte lyses, releasing the multiplied population of merozoites, which then infect new erythrocytes. Some parasites in erythrocytes mature into reproductive gameotcytes that are ingested by a feeding mosquito. In the insect gut, the gametocytes develop into oocysts that grow, rupture and release sporozoites that migrate to the mosquito's salivary glands, thus completing the cycle.
Clinical disease occurs when parasites invade and replicate within host erythrocytes, a process which may lead to life-threatening complications, including severe anemia, splenic rupture, cerebral malaria, respiratory distress, and/or renal failure. Morbidity and mortality result during the asexual development and replication of P. falciparum or P. vivax parasites within erythrocytes (Miller et al., 2002, Nature 415: 673-679). While malaria is generally curable if diagnosed promptly and treated correctly, and there are medications available for prophylactic treatment, malaria remains a leading cause of death and disease in many developing countries. In addition, drug-resistant malaria strains are increasing. Thus, the need for a effective malaria vaccine is high.
The intraerythrocytic parasites are somewhat shielded from many cell-mediated and antibody-mediated immune effector mechanisms, and naturally acquired immunity is slow to develop. When the intracellular parasite matures and the host erythrocyte is lysed, the merozoites released are accessible to serum immunoglobulins before they invade new red blood cells (RBCs). While neutralization of free merozoites can occur, plasmodial parasites have also evolved mechanisms to avoid invasion-inhibiting antibodies. There are several alternate invasion pathways that depend on complex interactions between sets of several merozoite proteins and several host erythrocyte receptors (Barnwell et al., 1998, Invasion of vertebrate cells: erythrocytes, p. 93-120. In I. W. Sherman (ed.), Malaria: parasite biology, pathogenesis, and protection. ASM Press, Washington, D.C.; Berzins, 2002, Chem. Immunol. 80:125-143; Dolan et al., 1990, J. Clin. Invest. 86:618-624; Hadley et al., 1987, J. Clin. Invest, 80:1190-1193; Mitchell et al., 1986, Blood 67:1519-1521; Sim et al., 1994, Science 264:1941-1944). This redundancy of invasion pathways enables invasion to occur, even if one receptor-ligand interaction is blocked. In addition, merozoite-neutralizing antibodies are often strain specific due to a significant degree of polymorphism in many merozoite surface antigens (Barnwell et al., 1998, Invasion of vertebrate cells: erythrocytes, p. 93-120. In I. W. Sherman (ed.), Malaria: parasite biology, pathogenesis, and protection. ASM Press, Washington, D.C.; Berzins, 2002, Chem. Immunol. 80:125-143; Holder, 1996, Preventing merozoite invasion of erythrocytes, p. 77-104. In S. L. Hoffman (ed.), Malaria vaccine development: a multi-immune response approach. ASM Press, Washington, D.C.).
Several malaria vaccine strategies, which target pre-erythrocytic surface proteins, liver stage antigens and/or blood stage antigens, are currently being pursued (Heppner et al., 2005, Vaccine 23: 2243-2250; Mahanty et al., 2003, J. Exp. Biol. 206: 3781-3788; Richie et al., 2002, Nature 415: 694-701). The goal of blood-stage vaccines is to reduce parasite load and/or prevent life-threatening complications of malaria once parasites are replicating within red blood cells (RBCs). The single, most feasible strategy for blood-stage malaria is to immunize the host with subunit vaccines that induce high titers of antibodies that neutralize extracellular merozoites and prevent invasion of RBCs (Berzins, 2002, Chem. Immunol. 80: 125-143; Galinsky et al., 2005, pp. 113-168, In: I. W. Sherman (ed), Molecular Approaches to Malaria, ASM Press, Washington D.C.; Holder, 1996, pp. 77-104. In: S. L. Hoffman (ed), Malaria vaccine development: a multi-immune response approach, ASM Press, Washington D.C.; Mahanty et al., 2003 J. Exp. Biol. 206: 3781-3788). However, the multiple receptor-ligand interactions and alternate redundant pathways involved in merozoite invasion of RBCs, combined with the polymorphism of vaccine candidate antigens, present a challenge for vaccine design (Berzins, 2002, Chem. Immunol. 80: 125-143; Galinsky et al., 2005, pp. 113-168. In: I. W. Sherman (ed), Molecular Approaches to Malaria, ASM Press, Washington D.C.; Gaur et al, 2004, Int. J. Parasitol. 34: 1413-1429).
P. falciparum merozoite surface protein-1 (MSP-1; gp195) emerged during the 1980's as a viable blood-stage vaccine target. MSP-1 is an abundant component of the merozoite surface coat, is conserved across plasmodial species and is essential for parasite growth (Berzins, 2002, Chem. Immunol. 80: 125-143; Galinsky et al., 2005, pp. 113-168. In: I. W. Sherman (ed), Molecular Approaches to Malaria, ASM Press, Washington D.C.; Gaur et al, 2004, Int. J. Parasitol, 34: 1413-1429; Holder, 1996, pp. 77-104. In: S. L. Hoffman (ed), Malaria vaccine development: a multi-immune response approach, ASM Press, Washington D.C.; O'Donnell et al., 2000, Nat. Med. 6: 91-95). During schizont maturation and segmentation, MSP-1 is synthesized as a ˜195 kDa precursor protein that is proteolytically processed to form a multi-subunit complex expressed on the surface of merozoites (Holder et al., 1984, J. Exp. Med. 160: 624-62; Lyon et al., 1986, Proc. Natl. Acad. Sci. USA 83: 2989-2993; McBride et al., 1987, Mol. Biochem. Parasitol. 23: 71-84). MSP-142 refers to the 42 kDa, GPI-anchored component in the C-terminal portion of the protein which results from proteolytic processing. Subsequent additional cleavage near the time of invasion yields a 19 kDa, C-terminal domain, called MSP-119, on the merozoite surface (Blackman et al., 1990, J. Exp. Med. 172: 379-382). MSP-119 contains two highly-conserved, epidermal growth factor (EGF)-like domains, which are targets of protective antibodies and which are the major focus of the MSP-1 vaccine development effort (see, for instance, Burns et al., 1989, J. Immunol. 143: 2670-2676; Darko et al., 2005, Infect. Immun. 73: 287-297; Egan et al., 1995, Infect. Immun. 63: 456-466; O'Donnell et al., 2001, J. Exp. Med. 193: 1403-1412). It has been demonstrated that it is the conserved spatial structure of the MSP-1 EGF-like domains, however, and not their primary amino acid sequence, that is essential for parasite growth (O'Donnell et al., 2000, Nat. Med. 6: 91-95).
Vaccines based on the two major alleles of P. falciparum MSP-142 (PfMSP-142) are currently in clinical trials (Angov et al. 2003, Mol. Biochem. Parasitol. 128: 195-204; Ockenhouse et al., 2006, Vaccine 24: 3009-3017; Stoute et al., Vaccine, Epub 2005 Dec. 7). MSP-142 consists of the N-terminal component, MSP-133 and the C-terminal component, MSP-119. The MSP-133 processed fragment does not appear to be a primary target of neutralizing antibodies but can provide a source of parasite-specific, T cell epitopes. One problem has been the relatively low immunogenicity of PfMSP-142-based vaccines in non-human primates and in human subjects. To increase immunogenicity, P. falciparum MSP-1 (PfMSP-1) subunit vaccines formulated with different adjuvants have been tested in non-human primates. However, no adjuvants tested have enhanced PfMSP-1 immunogenicity to the desired level (Burghaus et al., 1996, Infect. Immun. 64: 3614-3619; Chang et al., 1996, Infect. Immun. 64: 253-261; Darko et al., 2005, Infect. Immun. 73: 287-297; Kumar et al., 1995, Mol. Med. 1: 325-332; Kumar et al., 2000, Infect. Immun. 68: 2215-2223; Stowers et al., 2001, Infect. Immun. 69: 1536-1546). Phase I safety and immunogenicity trials of PfMSP-142 formulated with AS02A (GlaxoSmithKline Biologicals); an oil-in-water emulsion containing both QS21 and 3-deacylated monophosphoryl lipid A, have been completed in malaria-naïve US volunteers and semi-immune Kenyan adults (Ockenhouse et al., 2006, Vaccine 24: 3009-3017; Stoute et al., 2007, Vaccine, 25:176-184, Epub 2005 Dec. 7). The immunogenicity data and the relatively low activity of elicited antibodies in growth inhibition assays suggest that further improvements will be required.
Thus, there is a need in the art for a vaccine with improved immunogenicity and protective efficacy, and in particular, a malaria vaccine. The present invention addresses this need.